Welcome aboard, Molly! Celebrating your birthday (and your husband's!) onboard the Wonder will be so much fun. I celebrated my birthday on the Wonder last year and it was one of the best ones yet!
Disney Cruise Line is happy to provide a Pack n' Play for guests who need one, so be sure to add that note on your reservation so they have one ready for you. Each evening your stateroom host will set that up for you as well as break it down the following morning, so that you have more space during the day. However, if your little one still needs to take a nap, you could ask the stateroom host to leave it set up if you'd prefer.
If the cost difference for the sailing you're considering isn't that much between the Deluxe Oceanview stateroom and the Navigator's Verandah stateroom, I'd highly suggest booking the Navigator's Verandah. I say this because with a little one who will probably be going to bed much earlier than the two of you are ready to turn in, you'll love having the verandah to enjoy while your son sleeps. Then, you won't find yourself sitting on the bed whispering so as not to wake your son, but can actually be in a separate space where you can chat and enjoy the ocean views and fresh air! Your own verandah is also a nice place to relax and read a book while your son naps in the afternoon!
One last thought to leave with you: Check out
Palo, the Adult Exclusive restaurant onboard. What a wonderful place this would be to celebrate your birthdays with a romantic dinner! There is an additional cost of $20 per person there, but I promise it is well worth it! Don't forget to book time in Flounder's Reef Nursery ahead of time for your son, as that fills up quickly!
Have fun as you continue planning, Molly! Let me know if you have other questions as you continue to plan!